Generation VI onwards

Characteristics

Defense

The Dark type, along with the Steel type, was introduced in Generation II in order to balance against Psychic type, so it is immune to Psychic attacks. However, this immunity is nullified by Miracle Eye. Dark Pokémon have a single Ground weakness if their other type is Poison, which nullifies all of Dark type's weaknesses.

Offense

Dark-type attacks are resisted by Fighting and Dark, so using them along with Fairy attacks will optimize neutral coverage.

Contest properties

In Contests, Dark-type moves are typically categorized as Clever moves.

Pokémon

As of Generation VII, there are 54 Dark-type Pokémon or 6.7% of all Pokémon (counting those that are Dark-type in at least one of their forms, including Mega Evolutions and Alola Forms), making it the 6th rarest type.

In the TCG

Introduced in the Neo Genesis set, Darkness-type Pokémon in the TCG are generally weak to Fighting with resistances to Psychic. Darkness-type Pokémon can be strong against Psychic, whilst Fairy Pokémon can resist this type.

Trivia

Generation V introduced the most Dark-type Pokémon of any generation, with 16, while Generation VII introduced the fewest, with only two.

Generation IV introduced the most Dark-type moves of any generation, with 11, and Generation VI introduced the fewest (excluding Generation I) Dark-type moves, with three.

In Generation II, the Dark type saw one type change in a move (Bite), but no change in a Pokémon. The opposite is true for the Steel type, which saw one type change in a Pokémon family (Magnemite and Magneton), but no change in a move.

This makes the Dark type the only type added after Generation I not to have a previously introduced Pokémon have its type changed.

Prior to Generation VI, Dark/Ghost-type Pokémon normally had no weaknesses (excluding Fighting under immunity-negating conditions such as Foresight or Scrappy), as the resistances of the Dark type cover the weaknesses of Ghost, and vice-versa. Introduced in Generation VI, the Fairy type is super effective against Dark and not resisted by Ghost.

All moves that fail unless used by a certain species or form of Pokémon are Dark type.